Silver halide emulsions containing metal chelates as stabilizers



United States Patent O Int. Cl. G03c 1/34 U.S. Cl. 96109 4 Claims ABSTRACT OF THE DISCLOSURE Photosensitive, colloid-silver halide emulsion layers of improved stability having in intimate association therewith a metal chelate or complex of the general formula:

where M is zinc, silver, cadmium, or mercury; R is an alkyl, aralkyl, aryl or a hydrocarbyl (heterocyclic) radical; R can be the same as R or hydrogen; and n is a positive integer of 1 or 2.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to photographic silver halide emulsion layers and elements with improved stability and improved ratios of speed to fog.

Description of the prior art It is well known that photographic emulsion layers tend to lose sensitivity upon storage. When these layers are exposed and developed, small amounts of the silver salt are reduced in areas which have not been exposed irnagewise. This is known as fog.

Fog, when encountered, often results from the storage of the photographic elements at relatively high humidity and temperature. Often it extends throughout the entire emulsion but in severe cases it is more evident in some areas than others. Fog also depends on the emulsion and the development conditions. To determine fog charac-' teristics of a photosensitive emulsion, it is an industry practice to submit the photographic emulsion to accelerated aging tests to determine loss of speed and incerase in fog. Antifoggants are generally introduced into the photosensitive element to give maximum contrast and speed during processing. Such antifoggants may be: cadmium chloride, zinc chloride, etc. and others as disclosed in U.S. 2,839,405. These are generally inorganic salts and rather large quantities are required per mole of silver halide. Commonly, the organic antifoggant agents are compounds containing doubly bonded sulfur or an SH linkage. These organic antifoggants are often added as a separate stabilizing bath during processing as is disclosed in U.S. 2,453,346. Some of these same labile sulfur compounds also serve as emulsion sensitizers as is described in U.S. 3,144,336.

It has now been found that zinc, silver, cadmium and mercury when complexed or chelated with dithiocarbamates which are modified by alkyl, aryl, or heterocyclic radicals, as indicated below, produce a stabilizing effect on photosensitive emulsions when only small quantities,

e.g., 1 to 50 mg./mole of silver halide, are present in the emulsion.

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SUMMARY OF THE INVENTION The photographic element of this invention comprises a support coated with a light-sensitive, macromolecular water-permeable organic colloid-silver halide emulsion layer stabilized with a metal chelate or complex of the formula given below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS When practicing a preferred embodiment of this invention, photosensitive, gelatino-silver halide is prepared in the conventional manner including the steps of coagulation and washing. The emulsion is then preferably redispersed in a binder, generally gelatin, and digested. The stabilizing metallic organic salts of this invention are preferably added to the emulsion prior to digestion but may be in a layer contiguous with an emulsion layer, e.g., in an antiabrasion layer. These salts, including the preferred salt zinc dibenzyldithiocarbamate, are added to the emulsion in concentrations of 1 to 50 mg./mole of silver halide, preferably 5 to 25 mg./mole of silver halide.

The metal chelates or complexes useful in this invention are characterized by the general formula:

where M is Zn, Ag, Cd or Hg; R is alkyl of 1-4 carbons, aralkyl, e.g., benzyl and naphthylmethyl; aryl, e.g., phenyl and naphthyl or hydrocarbyl (heterocyclic), e.g., piperazine; R is the same as R or hydrogen; and n is 1 or 2. Suitable metal chelates include zinc, silver, cadmium or mercury chelates of dibenzyldithiocarbamate, dimethyldithiocarbamate, dibutyldithiocarbamate, piperazinodithiocarbamate, diphenyldithiocarbamate, etc. Whether it is one or two depends upon the stability of the particular chelate formed and the rapidity with which equilibrium is attained between the metal ion and the ligand. A discussion of equilibrium and chelate characteristics is found in Dwyer and Mellor, Chelating Agents and Metal Chelates, Academic Press, New York (1964).

To prepare the metal chelates and complexes useful in this invention, many commercially available chelating agents can be used, such as ammonium dibenzyldithiocarbamate. Conventional organic synthesis procedures can be used to obtain any of the modified dithiocarbamates. To obtain the metal chelate, a conventional precipitation process can be used, i.e., precipitating the chelating agent, e.g., ammonium dibenzyldithiocarbamate, with a metal salt, e.g., zinc nitrate in a suitable solution, e.g water. Many of the metal chelates are commercially available.

The preferred organic colloid of the silver halide emulsion is gelatin but other natural or synthetic binding agents may be used. Suitable binders include water-permeable or water-soluble polyvinyl alcohol, polyvinyl ethers, etc. and other binders as disclosed in U.S. 2,276,322; 2,276,- 323; and 2,495,918.

The emulsion is a photosensitive gelatino-silver halide such as is employed for use in X-ray, cine, graphic arts, or portrait film. The silver halide crystal is silver chloride, silver bromide, silver iodide, or mixtures thereof. The specific concentration of the stabilizers of this invention will depend upon the type of photosensitive emulsion being used.

The emulsion, in addition to the stabilizing compounds of this invention may also contain sensitizers. Suitable sensitizers include gold salts, e.g., gold thiocyanate, and other metal salts as disclosed in U.S. 2,540,086. Conventional sulfur sensitizers containing labile sulfur, e.g., allyl isothiocyanate, allyl diethylthiourea, sodium thiosulfate, etc. may also be included.

These emulsions may also contain other known emulsion adjuvants such as the usual hardeners, i.e., chrome alum, formaldehyde, dimethylolurea, mucochloric acid, dialdehydes, e.g., succinaldehyde, glutaraldehyde; matting agents, plasticizers, toners, optical brightening agents, surfactants, covering power enhancing polymers, e.g., dextran, polymeric latex dispersions, e.g., polyacrylate esters,

etc.

The support for the photographic element is preferably polyethylene terephthalate but may be any of the usual supports including other plastic films, paper, etc. Useful supports include cellulosic esters, polyamides, polyesters, e.g., polyethylene terephthalate/isophthalate esters formed by condensing terephthalic acid and dimethyl terephthalate and mixtures with isophthalic acid or esters with propylene glycol, diethylene glycol, tetramethylene glycol or cyclohexane-l,4-dimethanol (hexahydro-p-xylene alcohol). The vinylidene chloride copolymer coated, oriented, polyester films of Alles, U.S. 2,779,- 684 are especially suitable.

The photosensitive element may also contain conventional antihalation layers as well as antiabrasion layers.

The photosensitive emulsions containing the emulsion stabilizers of this invention were tested by accelerated aging that is, the coated elements were placed in an oven for 7 days at 120 F. and 65% relative humidity. The speed and fog were then checked by standard sensitometric tests. The results of these tests are outlined in the various examples. The photosensitive elements of this invention were given X-ray exposures, developed in an X-ray developer, fixed, and washed. The results of these tests are summarized in the examples.

The invention will be further illustrated by, but is not limited to, the following examples wherein, unless otherwise indicated, aging was at room temperature.

EXAMPLE I A high speed gelatino-silver iodobromide emulsion containing 1.8 mole percent silver iodide and 98.2 mole percent silver bromide was made in the conventional manner and then coagulated and washed as disclosed in U.S. 2,489,341 wherein the silver halide and most of the gelatin were coagulated by an anionic wetting agent, sodium lauryl sulfate, using an acid coagulation environment. Following the washing step, the emulsion was redispersed in water together with suflicient gelatin to give approximately a 1:1 silver to gelatin ratio. The emulsion was divided into equal portions prior to digestion and zinc dibenzyldithiocarbamate added to the various portions as disclosed in Table 1. To each of these portions there was added an organic sulfur sensitizer and a gold salt in amounts conventionally used to increased sensitivity. The emulsions were then digested for 0.60 hour. These emul sions were then coated in a conventional manner on a 0.004-inch thick copolymer coated polyethylene terephthalate film base as disclosed in U.S. 2,779,684. Samples of these coatings were then given a medical X-ray medium speed screen exposure at 70 kvp. and 4 milliamperes for 5 seconds through a V2 aluminum step wedge. A developer solution, Solution A, was then prepared as follows:

Potassium bromide Water to make 1000 ml.

The exposed strips were developed in Solution A for 3 min. at 68 F. Following development, the films were fixed in conventional X-ray fixer, washed and dried.

TABLE 1.SENSITOMETRIO PROPERTIES 7 days 120 F. Fresh 65% RH 11 Months Zinc dibenzyldithiocarbamate/mole silver Rel. Rel. Rel. halide, mg. speed Fog speed Fog speed Fog EXAMPLE II Example I was repeated except that the emulsion contained 1.2 mole percent silver iodide and 98.8 percent silver bromide. The emulsion was prepared, coated, exposed and processed as in Example I. The following is a summary of results.

TABLE 2.SENSIIROMETRIC PROPERTIES Fresh 1 Month 3 Months Zine dibenzyldithioearbamate/mole silver Rel. Rel. Rel. halide, mg. speed Fog speed Fog speed Fog None 100 0. 02 99 0. 04 99 0. 10 99 0. 02 100 0. 02 100 O. 04 97 0. 02 99 0. 02 97 0. 04

EXAMPLE III Example I was repeated except that a medium speed, panchromatic, gelatino-silver iodobromide emulsion containing 6.8 mole percent silver iodide and 93.2 mole percent silver bromide was prepared as in Example I. Prior to digestion, the emulsion was split into equal portions and the stabilizers added in the concentrations as indicated in Table 3. The emulsions were coated as in Example I except that the emulsions were coated on an acetate base having a conventional gelatin substratum.

Samples of the film were exposed in a Type 1B sensitometer and developed for 5 /2 minutes at 68 F. in developer Solution B. Developer Solution B was prepared as follows:

SOLUTION B Water ml 750 N-rnethyl-p-aminophenol hydrosulfate g 2.0 Sodium sulfite (anhyd.) g 98.0 Hydroquinone g 5.0 Borax g 2.0 Cold water to make 1 liter.

The following is a summary of results.

TABLE 3.SENSITOMETRIC PROPERTIES 7 days 120 13. Fresh 65% RH Addition/mole silver Rel. Rel. halide, mg. speed Fog speed Fog None 100 0.07 0.26 Silver diethyldithiocarbamate,#40 0.07 102 0.18 Cadmium dibenzyldithiocarbamate, #40--. 100 0. 06 97 0. 20 Mercury dibenzyldithioearbamate, #5 71 0.07 89 0.12

EXAMPLE IV Example I was repeated except that a blue-sensitive gelatino-silver iodobromide emulsion containing 3.2 mole percent silver iodide was prepared as in Example I. Prior to digestion, stabilizer salts were added to the emulsion in the amounts as indicated in Table 4. The photosensitive emulsion was then coated as in Example 1. Samples of the photosensitive elements were given a tungsten lamp exposure for 20 see. through a /2 step wedge. The films were then developed in developer Solution C for 5 /2 minutes at 68 F.

SOLUTION C Water ml 750 N-methyl-p-aminophenol hydrosulfate g 2.5 Hydroquinone g 2.5 Sodium sulfite (anhyd) g 50.0 Sodium carbonate (anhyd) g 20.0 Borax g 14.0 Potassium bromide g 0.5 Cold water to make 1 liter.

The following is a summary of results.

TABLE 4.SENSITOMETRIC PROPERTIES 7 days 120 Fresh F. RH

Rel.

Addition/mole silver bromide, mg. speed Fog Speed Fog None 100 0.06 127 0. 31 Zinc dibenzyldithiocarbamate,# 105 0.05 129 0.13 Cadmium dibenzyldithiocarbamate, #1 107 0. 04 130 0. 14 Mercury dibenzyldithiocarbamate, #10 104 0.05 140 0.20 Silver diethyldithioearbamate, #10 105 0. 04 136 0.19 Silver diethyldithiocarbamate, #100 74 0.04 113 0.08

EXAMPLE V A gelatino-silver iodobromide emulsion containing 1.2 mole percent silver iodide and 98.8 mole percent silver bromide was made as in Example I, then noodled and washed to remove impurities. The emulsion Was digested to optimum sensitivity with sensitizers as in Example I. The emulsion was thereupon coated on a subbed polyethylene terephthalate support as in Example I and allowed to dry. The photosensitive elements were then coated with a usual gelatin antiabrasion layer containing the emulsion stabilizing additives as indicated in Table 5. The stated antiabrasion layer was applied at a coating weight of 10 mg./gelatin dm. These overcoated photosensitive elements were then given a lead screen X-ray exposure at 200 kvp. of 4 milliamperes for 1 minute. They were developed for 5 minutes at 68 F. in developer Solution A. The stabilizing results of these additives are summarized in Table 5.

TABLE 5.SENSI'IOMETRIC PROPERTIES 7 days 120 Normal Rm.

6 As disclosed in Example V, the fog inhibitors of this invention may be incorporated in a colloid layer such as a gelatin layer in contact with the emulsion. These foginhibiting agents may also be incorporated in the various kinds of photographic silver halide emulsions.

The use of these metal chelates increases the resistance to developer fog and results in better aging fog performance. The metal chelates of this invention produce the added advantage that photosensitive elements can be stored for long periods of time with a minimum increase in fog and Without an appreciable loss in speed.

The embodiments of the invention in which an exclusive property or privilege is claimed as defined as follows:

1. A light-sensitive, water-permeable, macromolecular organic colloid-silver halide emulsion layer having in intimate association therewith a stabilizing amount of a metal chelate of the formula:

wherein M is Zn, Ag, Cd or Hg, R is an alkyl radical of 1-4 carbon atoms, aralkyl, aryl or piperazino, R is the same as R or hydrogen, and n is a positive integer of 1 or 2.

2. A layer according to claim 1 wherein the metal chelate is distributed throughout the layer in an amount of 1-50 mg. per mole of silver halide.

3. A layer according to claim 1 wherein the colloid is gelatin.

4. A layer according to claim 1 wherein the metal chelate is zinc dibenzyldithiocarbamate.

OTHER REFERENCES Beavers, Def. Pub. of Ser. No. 825,420 filed May 16, 1969, published in 866 O.G. 703, on Sept. 16, 1969.

NORMAN G. TORCHIN, Primary Assistant R. E. FIGHTER, Assistant Examiner U.S. Cl. X.'R. 961 10 

